I know that each transponder is linked to a unique 24-bit code assigned by ICAO. But to be seen by the Secondary surveillance radar the pilot set the squawk code (4 digit octal code) on the transponder as dictated by the Air Traffic Controller (ATC).
So, why do I need a ICAO 24-bit code? Has it any utility during navigation, or just during maintenance to identify the single transponder (and the aircraft associated)?
In short: the 24 bit address is critical on the technical layer of Air Traffic Control, but it is not used in the operational level. For identification of aircraft and association to flight plans, the air traffic controllers and the flight data processing systems rely mostly on the Mode A code and the aircraft ID (call sign) transmitted by the transponder.
The ICAO 24-bit aircraft address is an essential element of the way Mode S radar and TCAS work.
Before Mode-S radar, only modes A and C were used by civil ATC secondary radars.
Mode A and Mode C are unaddressed; every transponder receiving a Mode A or Mode C interrogation will reply*. This causes two problems, garbling and FRUIT.
Garbling is caused by the overlap of replies from multiple transponders. A transponder replies on 1090 MHz in the from of a number of pulses. For Mode A & C, there are at most 12 pulses between a set of framing pulses. When multiple aircraft reply to the same interrogation, these pulses mix up and it becomes difficult to find out which transponder sent which pulse. This leads to false altitudes or false squawk codes.
FRUIT (False Replies Unsynchronised to Interrogator Transmission) is caused by replies that are triggered by other radars. When multiple radars operate in an area, transponders can be quite busy and the radar is easily mislead by a reply to another radar. In addition to the timing (range) being wrong, the reply can be Mode A (squawk code) while the radar interrogated Mode C (altitude). There is no way to see what kind (A or C) reply was sent.
To overcome these problems of Mode A/C, Mode S was introduced. Mode S is quite different from Mode A/C and requires a more sophisticated radar and transponder. It uses the 24 bit address to distinguish responses from various aircraft and to interrogate specific aircraft.
Most Mode S interrogations are addressed; the interrogation contains the unique 24 bit address of the aircraft it is interrogating. This reduces the probability of garbling since other aircraft will not reply
Unaddressed Mode S interrogations will solicit replies that contain the address of interrogator (radar). This prevents FRUIT since the radar can verify that the reply is correctly addressed.
Unlike Mode A/C which does not have any error checking, a Mode S reply contains a 24 bit CRC check code.
Mode S Altitude and identity (squawk) replies have their own identifier so they can be distinguished.
Mode S radars can be operated in clusters and coordinate between them who is interrogating which aircraft at which time. This further reduces the message load and garbling / fruit probability.
For TCAS, the Mode S address is used to distinguish replies for various aircraft in the vicinity of the own aircraft. The TCAS transponder interrogates omnidirectionally (in all directions) and get's replies from all direction. Again, this is very susceptible to Garbling and FRUIT. Whilst TCAS can work with Mode A/C, for its reliability and coordination of resolution advisories, the 24-bit address is crucial.
The 24-bit address is used primarily in the low-level radio protocol between the surveillance radar and the transponder. The stationary radar encodes this address in its radio ping to specify that it wants a reply from that particular transponder, and all other transponders in the air will then not flood the downlink frequency with replies of their own.
This is what the S in "mode S" stands for: selective interrogation of transponders.
(From time to time a mode S radar will also issue pings that invite every transponder that hears it to reply, in order to learn of new aircraft entering its range of vision. But this can happen much less often than the everyone-respond pings of the old mode A/C protocol).
The 24-bit address is generally not used in higher layers of the system than the physical access layer; these use either mode A squawk codes or alphanumeric callsigns (aircraft ID) that are given in the Mode S response.
24-bit addresses are also used for ADS-B broadcasts -- partially because they reuse the Mode S data format where it is already present; partially because it is a more permanent identifier than callsigns or squawk codes, and therefore is useful for ground equipment to figure out which broadcasts come from the same aircraft.
The ICAO 24-bit code is used by Mode-S Transponders. Secondary Surveillance Radar (SSR) can query Mode-S transponders to not only send the Squawk Code and Altitude Information, but also the callsign or registration set in Mode-S transonders. This way you are able to use non-discreet codes within an airspace and still know which aircraft is which.
Example: In the German Langen Airspace (West of Germany), VFR traffic is assigned SQ Codes 3704 or 3703 when they call in for Flight Information Service. This way, the FIS specialist immediately knows which aircraft are on his frequency and can call them up, but he can still call them by the registration or callsign, as the Mode-S transponder not only transmits the squawk code (Mode-A), but also the registration and callsign (Mode-S). Some countries also use the Mode-S Downlink capabilities to correlate flightplans to aircraft targets and assign the code 1000. This frees up discreet squawk codes.
Related questions:
Is there a relation between the Mode 3/A Code and the ICAO aircraft address?
